The present invention relates to a fuel channel for a boiling water nuclear reactor and, more particularly, to an expanded fuel channel that reduces the number of individual components and creates an increased flow area at the top of the channel.
A conventional fuel bundle assembly in a boiling water nuclear reactor includes a lower tie plate, an upper tie plate and a matrix of sealed fuel rods supported between the upper and lower tie plates. The fuel rods contain nuclear fuel pellets in sealed containment for supporting a required critical reaction for the generation of steam. One or more coolant rods is included in the matrix of the fuel rods and is also supported between the upper and lower tie plates. A fuel channel surrounds the tie plates, fuel rods and one or more coolant rods. This channel is generally square in cross-section and made of metal (preferably Zircaloy). During reactor operation, coolant water enters through the lower tie plate within the fuel channel and flows upwardly between the upstanding fuel rods. Water and generated steam exit through the upper tie plate. The channel confines the required moderator coolant flow to a flow path and is restricted between the tie plates.
A typical boiling water reactor channel has a constant internal flow area formed by a full length square cross-section with rounded corners (see FIG. 1). Variations of the uniform thickness concept are disclosed in commonly-owned U.S. Pat. No. 4,749,543. The patent discloses channel designs that have reduced average thickness in the upper portion of the channels corresponding to a lower pressure difference acting on the channel sides in the upper portion of the channel. In some instances, the axial variation of the channel average thickness increases water volume adjacent the upper part of the channel to provide increased neutron moderation, to minimize the steam void reactivity coefficient, and to provide a greater cold shut-down margin.
Typically, a fuel channel is a constant cross-section with channel spacers and a channel fastener added to properly position the channeled fuel in the upper guide plate of the reactor. It would be desirable to eliminate the need for separate channel spacers and a fastener guard and additionally to create additional flow area to reduce the pressure drop relative to current designs.